1. Field of the Invention
This invention relates to an apparatus and method for dry-forming fibrous structures, and more specifically, to an apparatus and method for individualizing and blending different length fibers from a laminate feed, and forming a feed mat in which the different length fibers are blended together.
2. Description of the Prior Art
Low basis weight nonwoven fibrous webs in the basis weight range of from about 1 to about 6 oz/yd.sup.2 are becoming extremely popular. Some of these webs are formed predominately of short-length cellulosic fibers less than 1/4 inch in length, and include a minor proportion, by weight, of longer reinforcing fibers having a length greater than 1/4 inch. These webs can be used by themselves, or in conjunction with other materials as substitutes for conventional textile fabrics in articles such as disposable diapers, sanitary napkins, industrial and household wipers, cosmetic pads and the like.
U.S. Pat. No. 3,862,472, issued to Norton et al, and assigned to Scott Paper Company, discloses a highly desirable method for dry-forming a low basis weight nonwoven fibrous web which includes short and long fibers blended together, as described in the preceding paragraph. In accordance with the method disclosed in the Norton et al. patent loosely compacted batts of the short and long fibers are directed into a working section in which the fibers are individualized from their respective batts and blended together to form a fibrous feed mat. The feed mat is directed into a web forming device in which the fibers are again separated into their individual components and redeposited on a forming surface in the form of a fibrous web having a basis weight in the range of from about 1 to about 6 oz/yd.sup.2. The fibrous feed mat has a basis weight at least 3 times the average basis weight of the low basis weight nonwoven fibrous web to be formed, and in all cases, has a basis weight greater than 12 oz/yd.sup.2.
In the Norton et al process the blending operation, in which the individual batts of long and short fibers are separated into their individual fibers components and blended together, has been a significant speed limiting factor in the formation of low basis weight fibrous webs. Specifically, devices of the general type employing a main drum and satellite worker rolls are usable to accomplish the blending operation at low speeds. However, at high-production speeds these prior art devices do not reliably process fibrous material in which the predominate fiber component, by weight, is less than 1/4 inch in length, such as wood pulp fibers and cotton linters. At high production speeds it is preferred to rotate the main drum at a peripheral speed in excess of 10,000 feet/minute.
One prior art device which is capable of a fairly high speed operation is disclosed in U.S. Pat. No. 3,641,628. However, that device is designed to process 100% textile-length fibers, and is not operable, in a reliable manner, to process fiber blends including short fibers under 1/4 inch in length. Moreover, the device disclosed in the U.S. Pat. No. 3,641,628 is designed to permit the main drum to be continuously rotated at a surface speed approaching only 9,500 feet/minute; not at a speed in excess of 10,000 feet/minute as is desired in the high speed formation of a fibrous feed mat including a blend of textile-length fibers and shorter-length fibers. When the main drum is rotated at a surface speed in excess of 8,000 feet/minute it acts as a pump to pull excessive quantities of air into the working section of the apparatus. The air which is pulled into the working section is not properly controlled to permit short fibers under 1/4 inch in length to be individualized and positively directed through the outlet of the working section for deposition on a forming wire. Specifically, a substantial portion of the air flow about the satellite worker and turner rolls is counter to the direction in which the fibers are moved by said satellite rolls as said fibers are being processed. This counter air flow tends to prematurely doff, or separate the blend of short and long fibers from the toothed peripheries of the satellite rolls. These doffed fibers intermingle with each other and form "ropes" of fibers in the working area. These "ropes" ultimately will jam the apparatus and thereby cause poor quality formation of a fibrous web on the forming wire. Moreover, the formation of "ropes" in the working section requires undesirable machine down time for cleaning out the apparatus. The longer length textile fibers are hooked by the teeth of the satellite rolls much better than the short length fibers under 1/4 inch in length. This better hooking of the long fibers prevents the counter air flow over the satellite rolls from prematurely doffing the textile-length fibers from those rolls.